The Fixation of Character in Organisms

Abstract

1. The construction of a natural classification of organisms is made possible only by the fact that certain characters of every individual are more conservative and less subject to variation than others during evolutionary development. 2. The explanation of conservatism propounded by the theory of natural selection is unsatisfactory since, so far as we are able to determine, characters which are most firmly fixed are in general those of least importance for survival. 3. From a study of phylogeny it is possible to formulate certain general principles of conservatism which are valid throughout more or less extensive groups of organisms. 4. The principal categories of conservative characters are those of number, position and plan. 5. Particular organs or regions of the body, throughout large groups of animals and plants, are less subject to change than others and hence are seats of primitive characters. 6. The early ontogenetic stages of animals and plants repeat those characters which were most conservative and firmly fixed in their ancestry. 7. Evolutionary advance and increase in differentiation tend to result in the decrease of variability. This is analogous to the loss of potentialities during ontogeny and is also comparable to the formation of habit. 8. Organic evolution is dependent on the action of two opposing factors: that of progressive fixation, which tends universally toward greater rigidity and conservatism in all characters during evolutionary advance; and that of natural selection, which tends to maintain or increase the variability of those characters important for survival by eliminating individuals where such characters have become so fixed that the organism fails to possess a necessary degree of adaptability. Natural selection is not concerned with harmless and trivial characters which consequently tend to become very conservative and are of much value in classification. 9. Such general principles of phylogeny as these, if thoroughly established and defined, will make possible the construction of a truly natural classification of organisms on a logical and. uniform basis. They also present a clearer conception of the general method of evolution than is set forth by the theory of natural selection alone. The writer is much indebted to Professor Herbert W. Rand, of Harvard University, for suggestions and information.